This invention relates to the production of pressure-sensitive, carbonless transfer sheets for use in combination with pressure-sensitive record sheet of the type whereby on application of pressure, a chromogenic material is transferred from the carbonless transfer sheet to the record sheet which then develops a visible image. More particularly, it relates to the production of a pressure-sensitive carbonless transfer sheet having a coating containing a chromogenic material, which coating is cured to a solid film by radiation means. For purposes of this application the term "chromogenic" shall be understood to refer to materials such as color precursors, color developers and other color formers. Additionally, the term CF shall be understood to refer to a coating normally used on a record sheet and the term CB shall be understood to refer to a coating normally used on a transfer sheet.
Carbonless paper, briefly stated, is a standard type of paper wherein during manufacture the backside of the paper substrate is coated with what is referred to as a CB coating which contains one or more color precursors generally encapsulated in solution form. At the same time the front side of the paper substrate is coated during manufacture with what is referred to as a CF coating, which contains one or more color developers. Both the color precursor and the color developer remain in the coating compositions on the respective back and front surfaces of the paper in colorless form. This is true until the CB side of one sheet and the CF side of a second sheet are brought into overlying relationship and when sufficient pressure, as by a typewriter, is applied to rupture the CB coating to release the color precursor solution. At this time the color precursor contacts the CF coating and reacts with the color developer therein to form an image. Carbonless paper has proved to be an exeptionally valuable image transfer media for a variety of reasons only one of which is the fact that until a CB coating is placed next to a CF coating both coatings are in an inactive state since the co-reactive elements are not in contact with one another. Patents relating to carbonless paper products are:
U.S. Pat. No. 2,712,507 (1955) to Green
U.S. Pat. No. 2,730,456 (1956) to Green et al.
U.S. Pat. No. 3,455,721 (1969) to Phillips et al.
U.S. Pat. No. 3,466,184 (1969) to Bowler et al.
U.S. Pat. No. 3,672,935 (1972) to Miller et al.
Generally, the commercially successful transfer papers utilize microencapsulated oil solutions of color precursors applied as an adherent coating to the paper substrate. The microcapsule containing coating compositions used to prepare these papers are aqueous based and generally contain a binder for the microcapsules.
Transfer papers have been developed in which the oil solution of the color precursor has not been encapsulated, but rather is dispersed in an aqueous or organic solvent which contains a gelatin or resinous component. These dispersions are then coated onto a paper substrate and then dried by heat to give a pressure rupturable film having minute droplets as opposed to microcapsules of the oil solution distributed therein. Such transfer papers and processes for making them are described, for example, in the following patents:
U.S. Pat. No. 2,374,862 (1945) to Green
U.S. Pat. No. 2,550,466 (1951) to Green et al.
U.S. Pat. No. 3,020,170 (1962) to Macaulay
U.S. Pat. No. 3,020,171 (1962) to Bakan et al.
U.S. Pat. No. 3,079,351 (1963) to Staneslow et al.
U.S. Pat. No. 3,305,382 (1967) to Oronik
A disadvantage of coated paper products such as carbonless copy paper stems from the necessity of applying a liquid coating composition containing the color forming ingredients during the manufacturing process. In the application of such coatings volatile solvents are sometimes used which then in turn require evaporation of excess solvent to dry the coating, thereby producing volatile solvent vapors. An alternate method of coating involves the application of the color forming ingredients in an aqueous slurry, again requiring removal of excess water by drying. Both methods suffer from serious disadvantages. The solvent coating method involves the production of generally volatile solvent vapors creating both a health and a fire hazard in the surrounding environment. When using an aqueous solvent system the water must be evaporated which involves the expenditure of significant amount of energy. Further, the necessity of a drying step requires the use of complex and expensive apparatus to continuously dry a substrate which has been coated with an aqueous coating compound. A separate but related problem involves the disposal of polluted water. The application of heat not only is expensive, making the total paper manufacturing operation less cost effective, but also is potentially damaging to the color forming ingredients which are generally coated onto the paper substrate during manufacture. High degrees of temperature in the drying step require specific formulation of wall-forming compounds which permit the use of excess heat. The problems encountered in the actual coating step are generally attributable to the necessity for a heated drying step following the coating operation.
In general, patents concerned with the production and application of liquid resin compositions containing no volatile solvent, which resin compositions are subsequently cured by radiation to a solid film are:
U.S. Pat. No. 3,551,235 (1970) to Bassemir et al.
U.S. Pat. No. 3,551,246 (1970) to Bassemir et al.
U.S. Pat. No. 3,551,311 (1970) to Nass et al.
U.S. Pat. No. 3,558,387 (1971) to Bassemir et al.
U.S. Pat. No. 3,661,614 (1972) to Bassemir et al.
U.S. Pat. No. 3,754,966 (1973) to Newman et al.
U.S. Pat. No. 3,772,062 (1973) to Shur et al.
U.S. Pat. No. 3,772,171 (1973) to Savageau et al.
U.S. Pat. No. 3,801,329 (1974) to Sandner et al.
U.S. Pat. No. 3,819,496 (1974) to Roskott et al.
U.S. Pat. No. 3,847,769 (1974) to Garratt et al.
U.S. Pat. No. 3,847,768 (1974) to Kagiya et al.
These compositions generally also contain a pigment or a dye. Such resin compositions are useful for protective coatings and fast drying inks. U.S. Pat. No. 3,754,966 describes the production of an ink releasing dry transfer element which can be used as a carbon paper or typewriter ribbon.
The novel liquid coating composition of this invention contains an oil solution of a chromogenic material in addition to a liquid radiation curable substance. Prior to the discovery of this invention, it was not known that oil solutions of chromogenic material could be dispersed without encapsulation in radiation curable coating compositions and retain their chromogenic properties after the resin is cured by radiation to a tack-free film. For purposes of this disclosure, a tack-free film is one which will separate cleanly from a cotton ball lightly pressed against the film. The cotton fibers will not adhere to the film surface.